You can go To Goa;I visited the goa in Dec 2015.The beaches are decorated like a bride.At that time;the famous Sunburn festival also get organized there.You can go to cruise for the parties.And away from the beaches is peaceful,clean,less crowded area i.e. South Goa.Hire A scooty and wander all tha goa with the help of google maps.In other options;you can go to Jaisalmer. Sunrise and sunset are really majestic.I visited there last month.(Done safari and camping there). You can also go to Rann of Kutch.As Rann utsav(biggest festival)is going to start from Nov-Feb. For the mountains and snowfall go to Shimla,Manali or less explored likeGangtok..
If you like less crowded area, to enjoy your solo trip..Then go for Kasol. If you traveling via Delhi then you can book your bus from Delhi ( Kashmiri Gate) to Bhuntar, then take a local bus from Bhuntar.
You can choose Pondicheri ( French colonial settlement ).. You can enjoy a number of old and large churches in Pondicherry most of which are built in 18th & 19th century.. You can explore the hole city..
If you like crowded place then Goa will be the best place to visit in the month of December. Mostly the Sunburn Festival of Goa. This year sunburn festival will begin on 29th December. Ticket cost approximately 3000/-